1. Field
This document relates to a zener diode and methods for fabricating and packaging same.
2. Description of the Related Art
In general, there is a disadvantage in a device having a low voltage-resistance out of semiconductor devices in that its life span is shortened due to static electricity or surge voltage generated during implementation of measurement or packaging process. As a protective device against the low voltage-resistance, a zener diode is used.
The zener diode is a diode using a reverse breakdown voltage, and a reverse breakdown of PN zener diode includes a zener breakdown occurring at a low threshold voltage, and an avalanche breakdown occurring at a higher threshold voltage.
The zener breakdown is configured in such a manner that, if an impurity of high concentration is introduced into a semiconductor layer, a charge depletion layer of narrow width is formed to result in generation of a high electric field even at a low voltage.
In other words, if the impurity of high concentration is doped, energy bands are entwined even at a reverse-biased low voltage such that an energy band of valence electro band of P type semiconductor layer forms a higher energy level than that of the conductivity band of N type semiconductor layer.
If the width of the charge depletion layer is narrow, electrons filled in P type valence electron band generate tunneling breakdown toward N type conductivity band resulting in a diode of a very low resistance with a high current flowing therein.
A diode so fabricated as to generate a zener breakdown using the afore-mentioned principle is called a zener diode. If a reverse voltage applied to the diode reaches the zener breakdown voltage, reverse current suddenly increases to a great extent, but terminal voltage remains constant such that the diode can be used as a constant voltage diode.
Meanwhile, if a reverse surge voltage occurs in a device, for example, a light emitting diode (LED) having a low voltage-resistant characteristic, an excessive charge flows into the semiconductor layer to damage or deteriorate the LED.
These problems can be further worsened if devices are fabricated on top of an insulation substrate. The surge voltage may rise up to thousands of voltages such that protective devices must be separately installed if internal voltage (allowable voltage) of the device is low.
Therefore, the zener diode used for diodes having a low voltage-resistant characteristic and having a predetermined breakdown voltage may be used in the form of a PN zener diode (zener breakdown generated only in reverse direction), or in the form of a zener diode having a bi-directional threshold voltage characteristic where two zener diodes are connected in series in the same polarity direction (PNP or NPN) to cause a zener breakdown to occur at both regions in the forward and reverse directions.
When a zener diode having a bi-directional threshold voltage characteristic is connected to a device such as LED, the zener diode may be connected to a device for improving voltage-resistance in parallel regardless of polarity because polarity of two terminals of the zener diode is identical.
Consequently, if a surge voltage occurs in a device of low voltage-resistance connected to the zener diode, an over-current of any kind does not flow toward the low voltage-resistant device but bypasses toward the zener diode having a low resistant value because a zener breakdown occurs near a zener voltage, thereby enable to protect the device.
FIGS. 1a through 1h are cross-sectional view describing a fabricating process of a zener diode according to the prior art.
First of all, top and lower mask layers (11.12) are formed with top and bottom surfaces of N type semiconductor substrate (10), the top mask layer (11) is selectively etched and allowed to be mutually spaced apart, and a pair of openings (11a. 11b) are formed exposing the N type semiconductor substrate (10). (FIG. 1a)
Next, referring to FIG. 1b, when P type impurities are introduced into the top and bottom surfaces of the substrate (10) and diffusion process is conducted, P type diffusion layers (10a. 10b) are formed on the N type semiconductor substrate (10) region exposed to the openings (11a. 11b) of the top mask layer (11).
The top and bottom mask layers (11.12) are formed with property-changed layers (11a. 12a), and surfaces of the diffusion layers (10a. 10b) are formed with isolation layers (10Aa. 10Bb). For example, in case of a source for diffusion process being boron (B), the isolation layers (10Aa. 10Bb) are boron glass layers.
Successively, the mask layers (11.12), the property-changed films (11a. 12a) and the isolation layers (10Aa. 10Bb) are removed. (FIG. 1c)
Now, an isolation film (15) formed with contact holes (15a. 15b) exposing the diffusion layers (10a. 10b) is formed on the top of the substrate (10). (FIG. 1d)
Lastly, a pair of electrode lines (16a. 16b) electrically connecting each diffusion layer (10a. 10b) are formed via the contact holes (15a. 15b). (FIG. 1e)
When a zener diode having a bi-directional threshold voltage characteristic is formed by the method described earlier, an inter-layer insulation film should be vapor-deposited, the inter-layer insulation film should be etched to form a contact hole for connecting the diffusion layers to the electrodes. However, if a contact hole is formed at a region unwanted by the contact hole forming process, a PN zener diode or a resistant body, not a zener diode having a bi-directional threshold voltage characteristic, is formed to lower a yield thereof.
There is another problem in that a current has to flow across a long diffusion layer to increase a zener impedance because the current flows from an electrode line to a diffusion layer and substrate, and flows through another diffusion layer and the other electrode line.
There is still further problem in that a diffusion layer not contacting an electrode line exists due to limitation of contact hole forming process, and the diffusion layer and the electrode line are not self-aligned to result in a deteriorated reliability.
There is still further problem in that if the contact hole is formed by using dry etching, the diffusion layer is damaged by the dry etching to result in a deteriorated characteristic of zener diode.
FIG. 2 is a conceptual diagram illustrating a phenomenon where a zener impedance value is increased in the prior art of FIGS. 1a through 1e. 
As mentioned earlier, in order to form the insulation film (11) and selectively etch the insulation film (11) and to form the contact hole (15a) where top of the diffusion layer (10a) is exposed, following the formation of the diffusion layer (10a), width (W1) of the contact hole (15a) should be smaller than that (W2) of the diffusion layer (10a), and a gap ‘d’ from a lateral wall of the contact hole (15a) and a marginal proximity of the diffusion layer (10a) should be maintained. The ‘d’ value is larger than a preset value because operational error and distribution of the diffusion layer (10a) should be taken into account.
Therefore, as the gap of ‘d’ is increased, the current flow at the diffusion layer is lengthened to thereby increase a zener impedance value in the zener diode fabricating process according to the prior art.
FIGS. 3a through 3d are cross-sectional views illustrating a fabricating process of a zener diode according to the prior art.
Top and lower mask layers (11.12) are formed with top and bottom surfaces of P type semiconductor substrate (10), the top mask layer (11) is selectively etched and allowed to be mutually spaced apart, and a pair of openings (11a. 11b) are formed exposing the N type semiconductor substrate (10). (FIG. 3a)
Next, referring to FIG. 3b, when N type impurities are introduced into the top and bottom surfaces of the substrate (10) and diffusion process is conducted, N type diffusion layers (10a. 10b) are formed on the P type semiconductor substrate (10) region exposed through the openings (11a. 11b) of the top mask layer (11).
At this time, the top and bottom mask layers (11.12) are formed with property-changed layers (11a. 12a) due to the diffusion process as illustrated in FIGS. 1a and 1b, and surfaces of the diffusion layers (10a. 10b) are formed with insulation films (10aA. 10bB).
Successively, the insulation films (10aA. 10bB) are centrally removed. (FIG. 1c) to expose the diffusion layers (10a.10b). (FIG. 3c)
Lastly, a pair of electrode lines (16a. 16b) electrically connected to each diffusion layer (10a. 10b) are formed. (FIG. 3d)
There is a problem in the process according to the prior art in that, although there is no evaporating process of inter-layer insulation films, the films property-changed at the diffusion mask used for inter-layer insulation films become a cause generating leaked current at the electrode lines, and degrading the characteristics.
There is another problem in that a current flows across the long diffusion layer as described in FIG. 1 of fabricating method to increase the zener impedance value, and if the contact holes are formed using the dry etching, the diffusion layers are damaged by the dry etching to deteriorate the zener diode characteristics.